1. Technical Field
The present invention relates generally to communication networks, and more particularly, to digital cross connect systems.
2. Related Art
Because new data, voice and imaging applications are causing a fundamental shift in the nature of network traffic, the network architecture is required to evolve to accommodate this change. Instead of being dominated by voice data as in the past, the network traffic will increasingly carry bursty high-speed data transmissions. User applications and new network technologies including frame relay, switched multi-megabit data service and asynchronous transfer mode (ATM) are driving the transport network toward the synchronous optical network (SONET). SONET is a new transport medium, designed to enable communications between central offices. It defines optical signals and synchronous frame structure for multiplexed traffic as well as for operations and maintenance procedures.
As a result of the new demands being placed on telecommunication networks, switching systems must be designed to rapidly and correctly switch signal paths so as to properly route a signal or series of signals. Digital Cross Connect Systems achieve this result. A digital cross connect system is a specialized type of high-speed data channel switch that is used in modern telecommunication networks. Digital cross connect systems differ from normal voice network switches in that voice network switches make switching changes in the network responsive to dialing instructions by the user or calling party. In digital cross connect systems, however, specific and separate instructions are used to control the network switching. The separate instructions are given independently of any calls being routed through the system. Stated differently, switching instructions and voice go together in normal voice networks while they are transmitted separately in networks utilizing digital cross connect systems. Switching commands for a digital cross connect system may be given either by an operator at a control console or a computer algorithm, or both may specify them.
Digital cross connect systems work integrally with modem signaling systems to transfer control information between elements of an Integrated Services Digital Network (ISDN) and of Signaling System Number 7 (SS7). The purposes of a signaling system is to transfer control information (signaling units) between elements in a telecommunications network. In the modern networks, the signaling units are transmitted in separate channels.
All service providers including exchange carriers, long distance carriers and competitive bypass carriers are increasingly using digital cross-connect systems. Existing digital cross-connect system architectures generally have been based on a single core approach in which all cross-connections are made through a single switching node or fabric. To handle layered signal structures used in today""s transport networks these single switching nodes have been connected in series.
A typical arrangement of digital cross connect systems for a given network includes having a plurality of matrix elements connected in a multiple stage arrangement. In a three stage arrangement, a given cross connect system includes O stage matrix elements, C stage matrix elements and T stage matrix elements, where each O stage can support a connection to each C stage, and where each C stage can support a connection to each T stage. Each matrix element is formed to allow an operator (or program) to selectively create signal paths by closing connections between lines that are coupled to signal controlled switches.
In a simple non-blocking matrix, an input stage matrix has two outputs for every input. A center stage matrix has an equal number of inputs to outputs, the number of inputs and outputs being equal to the number of outputs of the input stage matrix. The output stage matrix has twice the number of inputs to outputs in this arrangement, the number of outputs being equal to the number of inputs of the input stage matrix. The number of inputs for the output stage matrix is equal to the number of outputs of the input stage matrix.
Some prior art networks that utilize cross connect matrices include a primary as well as a backup cross connect matrix arrangement to provide redundancy in the event of a failure in a primary or main path. In one particular arrangement, a line card is used to receive a signal and to transmit it to the originating stages of the main and the backup cross connect matrices. In another arrangement, separate line cards receive the signal from main and backup transmission paths. One line card produces the received signal to a originating stage matrix for the main cross connect matrix arrangement while the other line card produces the received signal to the backup cross connect matrix arrangement. At the output end of the matrix arrangements, one line card receives the output signals from the terminating stage matrices of the main and backup matrix arrangements and selects one of the signals for outputting from the network.
The embodiment having two input line cards is advantageous in that back-up path cross connect switching exists to provide redundancy and to overcome primary or main path failures. One shortcoming, however, is that switching paths from a primary path to a back-up path at an output stage or line card often results in framing errors requiring a portion of a signal transmission to be regenerated. Sometimes, it is necessary to switch center stages in a primary matrix for maintenance. For example, it is sometimes necessary to switch center stages in a cross connect matrix to alleviate blocking for new connections. Under the current techniques, framing errors are often introduced when the center stage is switched in a cross connect matrix network.
One reason the framing errors exist is that each line card, in the embodiment utilizing two input line cards, receives its respective signals from different network paths, the primary and backup signals are not received in frame alignment. Accordingly, switching from the primary matrix to the backup matrix as a part of switching center stages of the matrix network results in the introduction of framing errors or delays that require a signal to be re-framed. What is needed therefore is a solution that allows for center stages to be switched in a configuration that incorporates two input line cards.
To overcome the shortcomings of the prior systems and their operations, the present invention contemplates an apparatus and a method for a digital cross connect system for switching center stages within the main and back-up matrix arrangements without losing downstream framing alignment. The method includes routing the primary signal through the back-up path matrix to the terminating line card instead of the back-up signal while also temporarily routing the primary signal through the primary matrix. Thereafter, the output signal from the back-up matrix is selected, the center stages are switched in the primary matrix and then the output signal from the primary matrix is reselected. Now, the backup matrix may be rearranged if desired.
Because the primary signal is being propagated through both the primary and the back-up path matrices, the output of the two paths are frame aligned. When the output is switched from the primary path to the back-up path, therefore, there are no framing errors encountered. Once the center stages have been successfully switched, then the primary signal is reselected from the primary (main) portion of the matrix network. The back-up path is rearranged so that signaling is conducted through the back-up path once again in the event that a failure occurs in the primary path.
Other aspects of the present invention will become apparent with further reference to the drawings and specification that follow.